Learn how London dispersion forces are created and what effect they have on properties such as boiling and melting points. Discover this weak intermolecular force and how it is one of the van der Waals forces.

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Learn how London dispersion forces are created and what effect they have on properties such as boiling and melting points. Discover this weak intermolecular force and how it is one of the van der Waals forces.

London Dispersion Force

You know that every atom and molecule has electrons and that these electrons are in constant motion. At any one instant in time, these electrons can be more towards one side of a molecule than another. When the electrons are concentrated more at one end of a molecule, that end becomes slightly negative. The other end, where the electrons are not as concentrated, becomes slightly positive. At this instant, this molecule is a temporary dipole. This dipole can encourage a nearby molecule to also become dipole because the negative side of the first molecule will cause the electrons to run away on the other molecule (since negative (-) detests negative (-)).

Now these two adjacent dipoles created from the movement of electrons are attracted to each other. This very weak intermolecular force is called London dispersion force. London dispersion force is the weak intermolecular force that results from the motion of electrons that creates temporary dipoles in molecules. The London dispersion force is sometimes called a 'van der Waals force.' Van der Waals force is a general term that describes any attractive intermolecular force between molecules and includes both the London dispersion force and the dipole-dipole force discussed elsewhere.

The London dispersion force causes the attraction of adjacent dipoles.

Weakness of Force

As you can imagine, this is an extremely weak force - much weaker than hydrogen bonding or any other intramolecular force. As I mentioned, this force works between all atoms and molecules. This is the only intermolecular force that works on noble gases and nonpolar molecules.

A London dispersion force works because of the movement of electrons. As you can imagine, the more electrons in the atoms, the stronger the force. Also, in a larger atom, the electrons are going to be filling the higher energy shells, which means they will be farther away from the nucleus and thus be less tightly held by the nucleus' attraction. The electrons farthest away are able to move more freely, creating a dipole easier. This is why the London force increases with increasing atomic mass.

Effects on Properties

Intermolecular forces affect the properties of a substance. The stronger the bonds between - and among - molecules in a substance, the greater that substance sticks together. For instance, helium, He, is a noble gas. The only intermolecular force it has between molecules is a London dispersion force. This force is very weak, so it doesn't hold those molecules together very strongly. That is why helium has such a low boiling point of -452° F. Another example is the nonpolar molecule methane, CH4. Because it is nonpolar, it only experiences London dispersion forces to keep it together in a liquid. Its boiling point is -263° F. Now look at NH3, ammonia. It has only one fewer hydrogen atom, but it is polar and experiences hydrogen bonding. Its boiling point is a much higher -28° F. Contrast this with water, H20, which is not only a polar compound, it also experiences hydrogen bonding and has some other special properties. Its boiling point is +212° F. The boiling point of hydrogen alone, without the attached oxygen, is over 600 degrees lower, at -423° F.

Helium only has the weak London dispersion force holding the molecules together.

The same but opposite is true for melting points. Substances are going to melt from a solid to a liquid easier if their individual molecules aren't so attracted to each other. The melting point of water is 32° F, a full 180 degrees difference between melting and boiling. For hydrogen alone the melting point is -434° F, only 11 degrees separating melting and boiling because there are such weak attractive forces acting on it. So as you can see, those intermolecular forces really affect the physical properties of substances.

Lesson Summary

There are several intermolecular forces acting on substances. The weakest of these forces is the London dispersion force, one of the van der Waals forces. It is the weak intermolecular force that results from the motion of electrons that creates temporary dipoles in molecules. This force is weaker in smaller atoms and stronger in larger ones because they have more electrons that are farther from the nucleus and are able to move around easier. The London dispersion force, like all intermolecular forces, affects the boiling and melting points of substances.

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